Iraq's prime minister has returned to Mosul and declared victory against the Islamic State group in the northern city following nine months of grueling urban combat.

Speaking Monday from a small base on the edge of Mosul's Old City, where heavy clashes have been underway for days, Haider al-Abadi said Iraqi forces had achieved victory "by the blood of our martyrs."

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He has made similar announcements in recent days despite ongoing clashes, and visited Mosul on Sunday to congratulate Iraqi troops.

Heavy fighting was still underway just a few hours before he spoke Monday, and it was unclear whether the last militants had been defeated.

U.S.-backed Iraqi forces launched a massive operation to retake Mosul in October, and in recent days they had confined the remaining militants in an area measuring less than a square kilometer (less than a mile).

The battle for Mosul killed thousands and displaced more than 897,000 people.